1. Field of the Invention
The present invention relates to an apparatus and a method of measuring a terahertz wave, and particularly, to an apparatus and a method of measuring a terahertz wave in time domain. Hereinafter, such as an apparatus will be also referred to as a THz TDS (Time Domain Spectroscopy) apparatus. More particularly, the present invention relates to a technique to suppress an offset current in a measuring apparatus.
2. Description of the Related Art
A terahertz (THz) wave is an electromagnetic wave with a frequency in an arbitrary frequency band within a range from 0.03 THz to 30 THz. This frequency range includes frequencies or bands of frequency at which characteristic absorption occurs due to structures or states of substances such as biological molecules. This characteristic absorption feature is useful to nondestructively analyze or identify a substance. Active efforts are currently being made to develop testing techniques using this feature. One example of an expected application is a safety imaging technique that can enhance or potentially replace an X-ray imaging technique. Another example of an application of terahertz waves is the development of a high-speed communication technique.
In time domain (a concept that allows the analysis of mathematical functions or physical signals with respect to time), terahertz waves generally have the form of a pulse with a width of sub-pico seconds. Due to the slow response that current electronics have with respect to the speed of THz waves, it is generally difficult to acquire such a pulse in real time. For this reason, a THz-based apparatus operating in the Time Domain Spectrum (hereafter “THz-TDS apparatus”) employs a sampling measurement technique using ultrashort pulsed light with a pulse width in the order of femto seconds. The sampling of the terahertz wave is achieved by adjusting a difference between a time at which excitation light arrives at a generating unit that generates the terahertz wave and a time at which the excitation light arrives at a sensor unit that senses the terahertz wave. For example, the time difference can be produced by a stage having a folded optical system (also referred to as an optical delay unit in the present description) provided in a propagation path of the excitation light. More specifically, the time difference is produced by adjusting the total round-trip length of the optical path of the excitation light in the folded optical system (see, for example, Japanese Patent Laid-Open No. 2008-20268). In many cases, the generating unit and/or the sensor unit are implemented using a photoconductive device including an antenna electrode pattern having a small gap formed on a semiconductor film.
Another obstacle in detecting a terahertz wave is that a signal of a terahertz wave output from the sensor unit is extremely low in strength. To handle such a weak signal, efforts have been made to develop techniques to increase the signal-to-noise ratio of measuring apparatuses. For example, it has been proposed to reduce background noise of a measuring apparatus by subtracting a signal that appears in a state where there is no terahertz wave from a signal obtained in a state where a solid-state image pickup device is irradiated with a terahertz wave. In this proposed technique, the sensor unit is realized using not a photoconductive device but a solid-state image pickup device (see, for example, Japanese Patent Laid-Open No. 2007-292600).
The technique disclosed in Japanese Patent Laid-Open No. 2007-292600 may be combined with the technique disclosed in Japanese Patent Laid-Open No. 2008-20268 such that the signal output from the photoconductive device is acquired in a state in which the photoconductive device is irradiated with the terahertz wave and the signal is also acquired in a state where no terahertz wave strikes the photoconductive device (to obtain background noise), and the difference between these two signals is determined. The signal output from the photoconductive device is in the form of a current signal, and thus a current-voltage conversion circuit (also referred to as a current detection unit in the present description) is generally used to detect the signal. To increase the signal-to-noise ratio of the measuring apparatus, the current detection unit may be configured to have a high current-to-voltage conversion ratio. However, a rated output is defined for the circuit depending on its circuit configuration, and the output of the circuit is limited by the rating. In particular, in a case where background noise (in the present description, a current signal corresponding to the background noise is also referred to as an offset current) is large, it is necessary to limit the current-to-voltage conversion ratio such that the output of the current detection unit is not saturated. That is, to achieve a further improvement in the signal-to-noise ratio of the measuring apparatus, the current-to-voltage conversion ratio for the signal component of the terahertz wave (obtained by subtracting the background noise) may be set to be as large as possible within a range that does not exceed the rating of the circuit.